Much
of the work on vegetation classification
comes from European and North American
ecologists, and they have fundamentally
different approaches. In North America,
vegetation types are based on a combination
of the following criteria: climate pattern,
plant habit, phenology and/or growth form,
and dominant species. In the current US
standard (adopted by the Federal Geographic
Data Committee (FGDC), and originally
developed by UNESCO and The Nature Conservancy),
the classification is hierarchical and
incorporates the non-floristic criteria
into the upper (most general) five levels
and limited floristic criteria only into
the lower (most specific) two levels.
In Europe, classification relies much
more heavily, often entirely, on floristic
(species) composition alone, without explicit
reference to climate, phenology or growth
forms. It often empahsizes indicator or
diagnostic species which separate one
type from another.
In
the FGDC standard, the hierarchy levels,
from most general to most specific, are:
system, class, subclass, group, formation,
alliance, and association. The lowest
level, or association, is thus the most
precisely defined, and incoporates the
names of the dominant one to three (usually
two) species of the type. An example of
a vegetation type defined at the level
of class might be "Forest, canopy
cover > 60%"; at the level of
a formation as "Winter-rain, broad-leaved,
evergreen, sclerophyllous, closed-canopy
forest"; at the level of alliance
as "Arbutus menziesii forest";
and at the level of association as "Arbutus
menziesii-Lithocarpus densiflora forest".
In practice, the levels of the alliance
and/or association are the most often
used, particularly in vegetation mapping,
just as the Latin binomial is most often
used in discussing particular species
in taxonomy and in general communication.
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